This invention relates generally to the computerized recognition of user inputs, and more particularly to equation recognition in a pen-based computer system.
A pen-based computer system typically comprises a small, hand-held computer where the primary method for inputting data includes a "pen" or stylus which "writes" upon the screen of the computer system. The screen is a dual-function display assembly which serves as both an input device and an output device. When operating as an input device, the display assembly senses the position of the tip of a stylus on the viewing screen and provides this positional information to the computer's central processing unit (CPU). Some display assemblies can also sense the pressure of the stylus on the screen to provide further information to the CPU. When operating as an output device, the display assembly presents computer-generated images on the screen.
The dual-function display assemblies of pen-based computer systems permit users to operate the computer as a computerized notepad. For example, graphical images can be input into the pen-based computer by merely moving the stylus on the surface of the screen. As the CPU senses the position and movement of the stylus, it generates a corresponding image on the screen to create the illusion that the stylus is drawing the image directly upon the screen, i.e. that the stylus is "inking" an image on the screen. Besides serving as a notepad, pen-based computers can provide a number of useful functions, such as serving as an address book, an appointment calendar, a to-do list, a calculator, etc.
Ink on the screen of a pen-based computer system is typically stored as a simple bit-map. Essentially, the only knowledge that the computer system has of the inked image is that certain pixels of the display are to be activated to create the inked image. Therefore a note, such as "3+4=" has no meaning to the system other than certain pixels of the screen are to be turned on or off to create the inked image of the equation.
It is often desirable to perform some level of recognition on the inked objects formed on the computer screen. In this way, additional meaning can be attached to the inked objects allowing the computer to manipulate the objects in a more intelligent fashion. Recognition systems for characters and words are fairly well known. The present invention is concerned with the recognition, solving, and editing of equations and the like.
It is known that the symbols or characters comprising a class of patterns (language) have relationships that can be defined in terms of a grammar (also known as syntactic methods). A grammar is a set of syntactic rules and symbols which define a language (or domain). Different grammars may have different types of rules and/or symbols. The language may be a spoken language, e.g. English or French, or it could be a computer language, e.g. Fortran or PASCAL. The syntactic rules define a method by which an expression may be identified as a valid or invalid expression in a language. In Computer Science, grammars have also been used to describe the logic of computing. Basic courses in automata theory use grammars to define finite and non-finite computing models.
With respect to pattern recognition, it is desirable to use syntactic methods when it is convenient to represent a pattern as a collection of one or more subpatterns. A subpattern is defined as a symbol or a group of related symbols. For a written language a symbol may be a letter of an alphabet or a mathematical operator or a graphical symbol. Syntactic methods are also desirable when the validity of a pattern depends on the relationships of its subpatterns. For example, for recognizing the validity of an equation, it may not be valid to have two mathematical operators being adjacent, even though on their own, each operator is a valid symbol in the grammar.
While a grammar defines the valid symbols and syntactic rules, a particular pattern is analyzed using a parsing process. Conventional methods of parsing are linear, i.e. top-down or bottom-up. A parsing process will determine whether a pattern is valid or invalid. The parsing process will determine validity via a lexical analysis (i.e. checking the validity of the individual subpatterns) and by determining that the syntactic rules are followed. If the pattern is valid, the parsing process will return a parsed representation of the pattern according to the syntactic rules. If the pattern is invalid, the parsing process may terminate and provide information as to why the pattern is invalid.
As noted above, in the art of pattern recognition, syntactic methods have been utilized. One such syntactic method is known as an attribute grammar. Attribute grammars are discussed in an article entitled "Attributed Grammar--A Tool for Combining Syntactic and Statistical Approaches to Pattern Recognition", Wen-Hsiang Tsai and King-Su Fu, published in IEEE Transactions on Systems, Man, and Cybernetics, pgs. 873-885, Vol. SMC-10, No. 12, December 1980. In an attribute grammar, semantic information of the patterns is combined with the syntactic rules to create a production rule. The syntactic rules establish a relationship among subpatterns. The semantic information is used to compute attributes of a pattern using the attributes of the related subpatterns. The attributes of a related subpattern may also be used to indicate the applicability of a production rule.
An attribute grammar is defined as a five tuple EQU G =(V.sub.T, V.sub.N, A, P, S),
where V.sub.T is a set of terminal symbols, V.sub.N is a set of nonterminal symbols, A is a set of attributes, P is a set of production rules, and S is the start symbol. A symbol is merely a representation of a pattern or subpattern within the grammar. A terminal symbol represents subpatterns that cannot be further divided (e.g. a letter in an alphabet), whereas a nonterminal symbol represents a subpattern that may be further divided. For each x.epsilon.V.sub.T .orgate.V.sub.N, the expression A(x) denotes the attribute values of x, although some of the attribute values may be undefined for a given symbol. Each production rule in P has two parts, the first part of the rule specifies a syntactic restriction among the symbols and the second part of the rule specifies a semantic restriction among the symbols. The syntactic part is of context-free form. The semantic part describes how the attribute values of the left-hand side symbol of the syntactic rule are computed in terms of the attribute values of the symbols on the right-hand-side. Alternatively, the semantic part can indicate under which conditions the syntactic rule applies. Formally, the syntactic pan of a rule is: EQU B.fwdarw.B.sub.1 B.sub.2 . . . B.sub.n
where B.sub.i .epsilon.V.sub.N .orgate.V.sub.T, for 1.ltoreq.i.ltoreq.n.
The semantic part of the rule is a set of mappings. There are as many mappings as the number of attributes for the nonterminal B. Each mapping computes the corresponding attribute value of B from the attribute values of B.sub.1 B.sub.2 . . . B.sub.n.
Known attribute grammars are limited to defining one-dimensional relationships between subpatterns. It has been recognized that for some applications it is desirable to define multi-dimensional relationships between the subpatterns. This may occur for example in the analysis of mathematical expressions or fractions. In a fraction, one integer value is above a fraction line while a second integer value is below the fraction line. The same can be true for sub-expressions of a mathematical expression. Here, the relationships between the symbols are both horizontal and vertical (i.e. two-dimensional).
A method for describing and analyzing patterns with 2-D relationships is described in a paper entitled "Syntax-Directed Recognition of Hand-Printed Two-Dimensional Mathematics", Robert H. Anderson, Interactive Systems for Experimental Applied Mathematics, pgs. 436-459, New York, Academic Press, 1981. The Anderson paper describes syntax rules for driving a parsing process. The syntax rules have corresponding conditions for positively identifying particular input. However, the method described requires extensive computing resources to perform a pattern recognition analysis. This is because the associated parsing requires the examination of an overly broad set of permutations of the subpatterns which comprise the pattern. In particular, the described method does not provide for the use of keywords or heuristic information within the parsing process.